(1) Field of the Invention
This invention relates to MR imaging apparatus using NMR (nuclear magnetic resonance), and more particularly to an MR imaging apparatus for effecting high-speed imaging based on GRASE (gradient and spin echo) technique.
(2) Description of the Related Art
Various MR imaging apparatus capable of high-speed imaging have been conceived heretofore. For example, an MR imaging apparatus is known which effects a pulse sequence for high-speed imaging called GRASE technique (U.S. Pat. No. 5,270,954, and K. Oshio and D. A. Feiberg "GRASE (Gradient and Spin Echo Imaging: A Novel Fast MRI Technique", Magnetic Resonance in Medicine 20, 344-349, 1991). The pulse sequence based on the GRASE technique is one combining the EPI (Echo Planar Imaging) technique, a type of high-speed imaging technique which generates gradient echo signals by switching the polarity of a gradient magnetic field, and the RARE (Rapid Acquisition with Relaxation Enhancement) which generates spin echo signals by using an excitation RF (Radio Frequency) pulse and refocus RF pulses.
The pulse sequence based on the GRASE technique in conventional practice will be described with reference to FIGS. 1A through 1E.
In this sequence, as shown in FIG. 1A, one excitation RF pulse 100 (also called 90.degree. pulse since it rotates the spin phase of protons 90.degree.) is applied, which is followed by a plurality of (three in this example) refocus RF pulses 101-103 (also called 180.degree. pulses since they rotate the spin phase of protons 180.degree.). Simultaneously with these RF pulses, as shown in FIG. 1B, pulses 110-113 are applied to form slice-selecting gradient fields Gs. Then, as shown in FIG. 1C, a pulse 120 is applied to form a dephasing gradient field Gr for disarraying the protons, which is followed by pulses 121-123 applied between the above RF pulses to form reading and frequency-encoding gradient fields Gr.
Further, as shown in FIG. 1C, each of these Gr pulses 121-123 is switched a plurality of times (four times in this example) between one 180.degree. pulse and the next 180.degree. pulse (101 and 102 or 102 and 103). This generates spin echo signals S3(SE1), S8(SE2) and S13(SE3) [at points of time corresponding to an interval between 90.degree. pulse 100 and 180.degree. pulse 101 multiplied by even numbers], as well as gradient echo signals S1(SE1), S2(GE2), S4(GE3), S5(GE4), S6(GE5), S7(GE6), S9(GE7), S10(GE8), S11(GE9), S12(GE10), S14(GE11) and S15(GE12).
As shown in FIG. 1D, pulses of phase-encoding gradient fields Gp are applied immediately before generation of the respective echo signals S1-S15. These Gp pulses are applied in amounts corresponding to phase encode amounts for causing data acquired from the echo signals S1-S15 to be arranged in a K space (also called a raw data space) as shown in FIG. 2A.
Specifically, the data acquired from spin echo signals SE1-SE3 are arranged in a middle region (low frequency region) II of the K space. The data acquired from gradient echo signals GE1, GE5, GE9, GE2, GE6 and GE10, and those acquired from gradient echo signals GE3, GE7, GE11, GE4, GE8 and GE12, are arranged in peripheral regions (high frequency regions) I and III of the K space, respectively. In each of the regions I, II and III, the data are arranged from top to bottom in the order of echo signal generation, i.e. from the positive high frequency region through the low frequency region to the negative high frequency region. The pulses of phase-encoding gradient fields Gp are applied in the amounts to realize the above arrangement.
To provide such phase encode amounts, as shown in FIGS. 1D and 2B, the pulse applied in the greatest amount is pulse 131a of phase encoding gradient field Gp which is applied immediately after the first 180.degree. pulse 101 and immediately before the first gradient echo signal S1(GE1). As a result, the data acquired from the gradient echo signal S1(GE1) is placed in the uppermost position (positive region) in the K space. The pulses 131b-131e of phase-encoding gradient fields Gp immediately preceding the echo signals S2(GE2), S3(SE1), S4(GE3) and S5(GE4), respectively, have an opposite polarity to the gradient field pulse 131a. The pulses 131b-131e all have the same amplitude which is smaller than that of the gradient field pulse 131a. Consequently, the data acquired from echo signals S2, S3, S4 and S5 are equidistantly arranged in the K space downward from the position of the data acquired from signal S1 (see FIG. 2B).
The phase-encoding gradient field pulse 131f applied subsequently serves the rewinding purpose, i.e. for zero resetting phase encode amounts added up prior to application of the next 180.degree. pulse 102. The phase-encoding gradient field pulse 132a applied after the second 180.degree. pulse 102 has a slightly smaller amplitude than the gradient field pulse 131a. Consequently, the echo signal S6(GE5) has a phase encode amount to be disposed in the K space immediately below the data acquired from echo signal S1(GE1). The gradient field pulses 132b-132e immediately preceding the echo signals S7-S10, respectively, have the same amplitude and polarity as the above gradient field pulses 131b-131e. Consequently, the data acquired from echo signals S7(GE6), S8(SE2), S9(GE7) and S10(GE8) are arranged in the K space downward from the position of the data acquired from signal S6, at intervals corresponding to the intervals at which the data from echo signals S1, S2, S3, S4 and S5 are arranged. Thus, the data from echo signals S7, S8, S9 and S10 are arranged in the K space immediately below the data from signals S2, S3, S4 and S5, respectively. Subsequently, a rewinding gradient field pulse 132f is applied.
The phase-encoding gradient field pulse 133a applied after the third 180.degree. pulse 103 has a still slightly smaller amplitude than the gradient field pulse 132a. The gradient field pulses 133b-133e have the same amplitude and polarity as the gradient field pulses 131b-131e (and gradient field pulses 132b-132e). Consequently, the data acquired from echo signals S11(GE9), S12(GE10), S13(SE3), S14(GE11) and S15(GE12) are arranged in the K space immediately below the data from signals S6, S7, S8, S9 and S10, respectively.
As described above, phase encode amounts are set so that data acquired from the spin echo signals free from phase errors due to non-uniformity of the static magnetic field and due to chemical shifts are arranged in the middle region II of the K space (the middle region II being a low frequency region having a substantial influence on the contrast of an image reconstructed by a Fourier transform of the K space). This provides the advantage that a blur is unlikely to occur to the reconstructed image, which is one type of artifacts caused by discontinuous phase encode amounts in the K space due to phase errors. Further, in the above sequence, the echo signals having the same place in the order of generation within the respective periods between the 180.degree. pulses are grouped together (as SGE1, SGE2, SSE, SGE3 and SGE4 in FIG. 2A). This arrangement eliminates the phase errors at the boundaries between the echo signals grouped together (though the phase errors remain at the boundaries between the echo signal groups) to diminish the chance of a blur occurring to the image.
In the conventional pulse sequence described above, however, a striking difference .DELTA.S in signal strength occurs at the boundaries between the groups (between SGE1 and SGE2, between SGE2 and SSE, between SSE and SGE3, and between SGE3 and SGE4) of data acquired from the grouped echo signals. That is, the echo signals S1-S15 have strengths as shown in FIG. 2C. This is due to the fact that, as shown in FIG. 1E, the echo signals S1-S2 gradually attenuate with time constant T2 and time constant T2* after the first 90.degree. pulse 100 (in which time constant T2 represents a transverse relaxation time (also called spin-spin relaxation time) occurring with the spin echo signals, and time constant T2* represents a transverse relaxation time occurring with the gradient echo signals and involving a faster attenuation due to non-uniformity of the static magnetic field than time constant T2). Thus, echo signals S1-S15 have strengths diminishing in the order of generation thereof. The data acquired from echo signals S1-S15 are arranged in the K space as shown in FIG. 2A. As seen in the phase encoding direction (vertical direction) of the K space, the signal strength changes sharply at the boundary between the data of signal S11 and the data of signal S2 (i.e. the boundary between SGE1 and SGE2), the boundary between the data of signal S12 and the data of signal S3 (i.e. the boundary between SGE2 and SSE), the boundary between the data of signal S13 and the data of signal S4 (i.e. the boundary between SSE and SGE3), and the boundary between the data of signal S14 and the data of signal S5 (i.e. the boundary between SGE3 and SGE4). This results in a disadvantage that, when an image is reconstructed by a Fourier transform of the data arranged in the K space as above, artifacts will blur the reconstructed image.